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Case Study TutorialWetting and Non-Wetting

Basics of Wetting

2

G

L

S

surface

contact line

bulk

Three phase contact (TPC) zone

3

Three phase contact (TPC) line

steel surface

droplet

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Three phase contact (TPC) line

steel surface

droplet

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Capillary pressure

Pe

Pi

PPP ei

21 R

1

R

1P

is the interfacial tension, R1 and R2 are the two principal radii of curvature

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Young equation

SGYLGSL cos

QY

gLG

gSL

gSG

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Hysteresis

Viscous flow:Hindered TPC (pinned)Non-slip

Ideal flow: Barriereless TPCFree slippage

Qr < QY < Qa

Qr

Qa

QY

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The TPC line resistance (hysteresis) is due to solid surface heterogeneities:

morphologic and/or energetic

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Morphologic heterogeneity

The intrinsic contact angle at a rough surface is different from measured one:

Wenzel, Cassie-Baxter, wicking models

"God created the solids, the devil their surfaces"

Wolfgang Pauli (1900-1958)

REAL SURFACES ARE ROUGH

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Form

WaveGroove

1 order

2 order

3 order4 order

Topometric characterisation parameters

according to DIN EN ISOflatness, waveness, roughness

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Morphologic heterogeneity

Cassie-Baxter

1fcosfrcos sessrough

-1

1

C

tg = rs

1 - f

cos rough

cos flat

0-1 1

Johnson & Dettre in “Wettability”, Ed. by John C. Berg, 1993

Wenzel

esrough cosrcos

Bico et al. wicking

)cos1(f1cos esrough

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Adhesion, viscous friction and contact line barriers have the same nature: van der Waals interactions

In the case of: - non-slip boundary conditionsviscous fluids - barrier contact line motion

- TPC angle hysteresis

In the case of: - free boundary slippageideal fluids - barriereless contact line motion

- no TPC hysteresis (Young Model)

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30 mm

30 mm

hydrophobic hydrophilic superhydrophobic

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Super-hydrophobicity

We learn from nature ...

... and want to mimic

- adhesives- coatings- în microelectronics

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Super-hydrophobicity

Wettability can be manipulated through

- changes in surface energy- changes in surface morphology/topography

(roughness, geometry)

CA = 90 - 120°

CA 150°

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Super-hydrophobicity

Structure of rough surfaces can be:

RegularIrregular (Random)Hierarchical (Fractal): flat

2Dfractal cos)l/L(cos

L and l are the upper (of several micrometers) and lower limit (particle diameter) scales of the fractal behaviour on the surfaceD is the fractal dimension

Surface modified by particles: Regular Structure

10 m

R = 200 nm R = 1 m R = 2.4 m R = 5 m

9069.13R

2

R3RR

S

SS

S

Sr

2

222

triangle

poresegmentsphere

geometric

actualS

Regular particle structure: no superhydrophobicity

The height roughness (not the roughness factor) influences wetting

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ah

a

1 Under what condition is the Wenzel regime more stable than the Cassie-Baxter regime?

Wenzel, 1936 Cassie-Baxter, 1944

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ah

a

1 Under what condition is the Wenzel regime more stable than the Cassie-Baxter regime?

a

h1

a2

ah2

areaprojected

arearealrs

Wenzel roughness factor

Wenzel CA YYsW cosa

h1cosrcos

Cassie-Baxter CA 1fcosfrcos YfCB

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ah

a

1 Under what condition is the Wenzel regime more stable than the Cassie-Baxter regime?

If the liquid touch only the top of the surface, then f = ½ and rf = 1

2

1cos

2

1cos YCB

Wenzel regime more stable if W CB

Ycos 1

ah21

Wenzel regime is always more stable if Y 90°

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ah

a

2 Under what condition can this surface become non-wettable, i.e. superhydrophobic with a ? CA 150°

CBcos 866.0150cos

2

1cos

2

1cos YCB 866.0

Ycos 732.0

Y 137 but

120Y